Massive Multiple-Input and Multiple-Output (MIMO) technology has attracted much attention in wireless communications, because it offers significant increases in data throughput and link range without an additional increase in bandwidth or transmit power. Both theoretical and measurement results indicate that massive MIMO is capable of significantly improving the Spectrum Efficiency, while simultaneously improving energy efficiency.
In Massive MIMO systems (also known as Large-Scale Antenna Systems, Very Large MIMO, Hyper MIMO, Full-Dimension MIMO and ARGOS) a base station is equipped with a very large number of antennas (e.g., hundreds or thousands) that are operated fully coherently and adaptively according to a multi-user MIMO scheme.
Considering such a multi-user MIMO scheme where a plurality of users are respectively located in a plurality of cells of a communication network, inter-cell interference (ICI) and intra-cell interferences degrades the performance of massive MIMO system.
The reduction of inter-cell interference (ICI) can be achieved by using properly configured precoders at the transmitter side (i.e. at the side of at least one base station), in other words by choosing carefully beamforming vectors, or precoding weights, before the transmission of the signal to multiple users.
Among the known beamforming techniques, the coordinated beamforming (CBF) requires only a modest amount of signaling overhead and, for the sake of implementation, is classically proposed to improve system performance.
Different structures of coordinated beamforming can be implemented. In particular, a hierarchical structure of coordinated beamforming is disclosed by A. Liu in “Hierarchical interference mitigation for massive MIMO cellular networks”, IEEE Trans. Signal Process., vol 62, no.18, pp. 4786-4797, September 2014.
More precisely, according to such a structure, explicitly, each evolved Node B (eNB) consists of two layers: on the one hand a first precoder, called a “cell-layer precoder” in the following of the disclosure, and a second precoder, called a “user-layer precoder” on the other hand.
In general, as disclosed by A. Liu in “Two-stage subspace constrained precoding in massive MIMO cellular systems”, IEEE Trans. Wireless Commun., vol 14, no. 6, pp. 3271-3279, June 2015, the user-layer precoder supports data transmission to the active user terminal (UEs) by exploiting the knowledge of the time-variant channel state information (CSI). As far as the cell-layer precoder is concerns, it exploits the remaining spatial degree of freedom for mitigating the ICI by relying only on the knowledge of the long-term variant channel state information (CSI).
Although coordinated beamforming (CBF) relying on hierarchical structure needs little signal backhaul overhead, there are two main drawbacks of the existing coordinated beamforming hierarchical-structure: on the one hand, these existing schemes of the prior art are not suitable for implementation in practical massive MIMO system. Indeed, in the existing hierarchical-structure coordinated beamforming schemes, clusters of user equipments (UEs) are formed, and user equipments (UEs) in the same cluster have the same long-term channel state information (CSI), which is impractical.
On the other hand, although the cell-layer precoder of existing hierarchical-structure coordinated beamforming (CBF) schemes is adjusted to reduce Inter-Cell Interference with low complexity, the user-layer precoder of the prior art is settled as ZF precoder, and considers power allocation issue in precoding, which may result in more intra-cell interference.
Therefore, there is a need for an alternative precoding scheme, which overcomes the drawbacks of the above-mentioned method and can offer improved performances in terms of power consumption of all evolved Node B (eNB).